1. Field of the Invention
The present invention relates to a semiconductor device having a circuit consisted of a thin film transistor (hereinafter, referred to as TFT) and a method of manufacturing the semiconductor device. For example, the present invention relates to an electro-optic device that is represented by a liquid crystal module, a light emitting device that is represented by an electroluminescence display device and an electronic device on which such a device is mounted as a part.
It should be noted that in the present specification, the term “semiconductor device” indicates a device in general capable of functioning by utilizing the semiconductor characteristics, and an electro-optic device, a light emitting device, a semiconductor circuit and an electronic equipment are all semiconductor devices.
2. Related Art
In recent years, a technology constituting a thin film transistor (TFT) using a semiconductor thin film (in the range from about a few nm to a few hundreds nm in thickness) formed on the substrate having an insulating surface has drawn attention. A thin film transistor is widely applied to electronic devices such as an IC, an electro-optic device or the like, and particularly, there is an urgent need to be developed as a switching element for an image display device.
Although as for applications utilizing such an image display device, a variety of applications are expected, particularly, its utilization for portable apparatuses has drawn the attention. At present, although many glass substrates and quartz substrates are utilized, there are defaults of being easily cracked and heavy. Moreover, the glass substrates and quartz substrates are difficult to be made larger in terms of conducting a mass-production, and these are not suitable for that. Therefore, the attempt that a TFT element is formed on a substrate having flexibility, representatively, on a flexible plastic film has been performed.
However, since the heat resistance of a plastic film is low, it cannot help lowering the highest temperature of the process: As a result, at present, a TFT is formed which has not so excellent electric characteristics compared with those formed on the glass substrates. Therefore, a liquid crystal display device and light emitting element having a high performance by utilizing a plastic film have not been realized yet.
If a liquid crystal display device or a light emitting device having an organic light emitting device (OLED) can be formed over a flexible substrate such as a plastic film, it can be obtained as a thin light-weight device and can be used in a display having a curved surface, a show window, etc. Use of such a device is not limited to use as a portable device, and the range of uses of such a device is markedly wide.
Moreover, since the transparency of plastic film to the light is lower than that of glass substrate, it is not based on the quality of the material and thickness of a plastic film which pass light, but there is also a problem that the transparency become worse slightly.